1. Field of the Invention
The present invention relates to a bearingless rotary machine capable of serving as an electric motor for rotating a rotatable body and a magnetic bearing for magnetically levitating and supporting the rotatable body, and more particularly to a bearingless rotary machine which can stably control the levitation of a rotor even when the rotor comprises an induction-type rotor having secondary conductors as current paths.
2. Description of the Related Art
There have been proposed various bearingless rotary machines having a cylindrical stator, a cylindrical rotor disposed in the cylindrical stator, and an exciting winding circuits connected to the stator for generating two rotating magnetic fields with different numbers of poles to impart a rotating force to the rotor and apply a positional control force for levitating and supporting the rotor in a given radial position.
The stator has windings for rotating the rotor and windings for positionally controlling the rotor. These windings are supplied with a three-phase alternating current or a two-phase alternating current to generate rotating magnetic fields with different numbers of poles in a certain relationship within the gap between the stator and the rotor for thereby locally distributing a radial magnetic attractive force to the cylindrical rotor.
When currents are supplied to the stator windings, they produce a rotating magnetic field having M poles and a rotating magnetic field having N poles. The rotating magnetic field having M poles will hereinafter be referred to as a "drive magnetic field", and the rotating magnetic field having N poles as a "positional control magnetic field". The drive magnetic field applies a rotating drive force to the rotor, and the positional control magnetic field is added to the drive magnetic field to locally distribute the radial force to the rotor for freely adjusting the radially levitated position of the rotor as with magnetic bearings. The M poles and the N poles are related to each other as follows: EQU N=M.+-.2
for thereby locally distributing the radial magnetic attractive force to the cylindrical rotor.
In this manner, the bearingless rotary machine operates as an electric motor which magnetically attracts the rotor to impart a rotating force to the rotor and also as a magnetic bearing for controlling the radially levitated position of the rotor to levitate the rotor out of contact with the stator. The above bearingless rotary machine dispenses with an electromagnetic yoke and a winding which make up a magnetic bearing which has heretofore been required to support the rotatable shaft of an electric motor, and hence has a reduced shaft length and has its high-speed rotation less limited by shaft vibrations. The bearingless rotary machine is also small in size and weight. The current flowing through the positional control winding and the current flowing through the drive windings develop respective flux distributions that develop a synergistic action equivalent to a magnetic bearing, which can produce a large control force with a much smaller current than would be required by a conventional magnetic bearing, resulting in a large energy-saving arrangement.
One type of rotor is an induction-type rotor in which a rotating magnetic field generated by a stator causes induced currents to flow in secondary conductors of the rotor to impart a rotating drive force to the rotor. While there are various induction-type rotors, a typical induction-type rotor is a squirrel-cage type rotor. The squirrel-cage type rotor has a number of metal conductor bars (secondary conductors) of low resistance disposed as current paths parallel to a rotatable shaft in a circular pattern concentric with the rotatable shaft, and metal conductor rings (end rings) interconnecting the opposite ends of the metal conductor rods. When the current paths (secondary conductors) cross a rotating magnetic flux generated by the stator windings, the secondary conductors of the rotor develop induced voltages to produce induced currents. The magnetic flux generated by the stator windings across the secondary conductors and the induced currents flowing through the metal conductor rods of the rotor act with each other to produce a Lorentz force, applying a rotating drive force to the induction-type rotor.
In the bearingless rotary machine, the drive magnetic field and the positional control magnetic field are generated in a mixed fashion by the stator winding currents (primary currents). Therefore, if an ordinary induction-type rotor (squirrel-cage type rotor) is employed, then currents induced by the drive magnetic field and the positional control magnetic field flow through the rotor current paths (secondary conductors). Since the rotating magnetic field having M poles impart a rotating force to the rotor, the bearingless rotary machine will not operate as an induction motor unless an induced current flows. When induced currents developed by the positional control magnetic field having N poles flow in the rotor current paths, a magnetic field is generated as a disturbance by the rotor currents in addition to the magnetic field generated by the stator windings. Consequently, the positional control magnetic field is not determined by only the magnetic field developed by the winding currents, so that the rotor cannot stably be levitated.